1. Field of the Invention
The present invention relates to a multiple speed reproduction technology for use in an 8 mm-tape video cassette recorder and, more particularly, to a method for controlling tracking of a head for multiple speed reproduction. An apparatus controlling tracking of a head for multiple speed reproduction is also disclosed.
The instant application is based on Korean Patent Application No. 94-14649, which is incorporated herein by reference for all purposes.
2. Brief Discussion of Related Art
An apparatus for reproducing a video signal recorded on a magnetic tape in a speed different from a recording speed thereof is disclosed in U.S. Pat. No. 4,568,986 to Furuhata et al., which issued on Feb. 4, 1986, and is entitled "Variable Speed Playback Controller For A Video Tape Recorder". According to this reference, pilot signals having four different frequencies are periodically, repetitively and selectively recorded in sequential tracks on a magnetic tape, and local pilot signals having the same frequencies as those of the recorded pilot signals are sequentially generated in a predetermined sequence. The pilot signal reproduced from the magnetic tape is frequency-converted according to the local pilot signal. If the magnitude of a resultant differential frequency signal is detected, a servo control is performed to consistently maintain the frequency of the detected differential frequency signal. Technology using four kinds of pilot signals similar to those found in U.S. Pat. No. 4,568,986 is shown in FIG. 1A.
The tracking control apparatus of FIG. 1A is for an 8 mm-tape video cassette recorder, which adopts an automatic track following (ATF) method for controlling tracking of a head using four kinds of pilot signals recorded on the respective tracks of a magnetic tape. The pilot signals used in the FIG. 1A apparatus are recorded on sequential tracks periodically and repetitively as shown in FIG. 2A. Also, the pilot signals are overlapped with video information to be recorded on each track. The respective frequencies of the pilot signals are as follows: f1=102.5 kHz; f2=118.9 kHz; f3=165.2 kHz and f4=148.6 kHz. A frequency difference value between the pilot signals on adjacent tracks is approximately 16 kHz or 46.1 kHz, that is, f.sub.2 -f.sub.1 =f.sub.4 -f.sub.3 =16 kHz and f.sub.4 -f.sub.1 =f.sub.3 -f.sub.2 =46 kHz. An operation of the FIG. 1 apparatus when the 8 mm-tape video cassette recorder performs a double speed operation will be described below with reference to FIGS. 2A through 2E.
Two heads, shown in FIG. 1B, operate alternately according to a head switching signal HEAD-SWP of FIG. 2B. That is, during an interval when head switching signal HEAD-SWP has a high level pulse, head A operates, while during an interval when head switching signal HEAD-SWP has a low level pulse, head B, operates. Head B is a multi-functional head which is usually used for double speed reproduction, and is located in a position substantially opposed to that of head A on a head drum as shown in FIG. 1B. Also, head B has the same azimuth characteristic as that of head A. The heads perform tracking of the magnetic tape at the normal double speed reproduction as shown in FIG. 2A. Thus, as shown in FIG. 2D, head A reads a playback pilot signal PB-PILOT having frequencies f.sub.4, f.sub.1 and f.sub.2 and head B reads a playback pilot signal PB-PILOT having frequencies f.sub.2, f.sub.3 and f.sub.4. The playback pilot signal PB-PILOT read from the magnetic tape by the heads is supplied to an ATF integrated circuit ATF-IC 11. Here, a microprocessor 13 generates a reference pilot signal REF-PILOT as shown in FIG. 2C. That is, microprocessor 13 generates a reference pilot signal REF-PILOT having frequency "f.sub.1 " when the frequency components of the playback pilot signal PB-PILOT are f.sub.4, f.sub.1 and f.sub.2, while microprocessor 13 generates a reference pilot signal REF-PILOT of a frequency of "f.sub.3 " when the frequency components of the playback pilot signal PB-PILOT are f.sub.2, f.sub.3 and f.sub.4.
The reference pilot signal has a reference level V.sub.ref of a constant voltage irrespective of a frequency of a signal during performance of the double speed reproduction. The reference level V.sub.ref is supplied to ATF integrated circuit 11. ATF integrated circuit 11 detects difference values between the frequency value of the reference pilot signal REF-PILOT and the frequency components contained in the playback pilot signal PB-PILOT, and generates a tracking error signal ATF-ER representing a quantitative ratio of the detected frequency difference values.
In more detail, ATF integrated circuit 11 detects frequency difference values of 46 kHz, 0 kHz and 16 kHz if the playback pilot signal PB-PILOT contains frequency components f.sub.4, f.sub.1 and f.sub.2 and the reference pilot signal REF-PILOT has frequency f.sub.1, and ATF integrated circuit 11 detects frequency difference values of 46 kHz, 0 kHz and 16 kHz if the playback pilot signal PB-PILOT contains frequency components f.sub.2, f.sub.3 and f.sub.4 and the reference pilot signal REF-PILOT has frequency f.sub.3. ATF integrated circuit 11 generates tracking error signal ATF-ER having a value smaller than predetermined reference level V.sub.ref when the detected frequency difference value is 16 kHz, and generates tracking error signal ATF-ER having a value larger than predetermined reference level V.sub.ref when the detected frequency difference value is 46 kHz. When playback pilot signal PB-PILOT read via head A has frequency components f.sub.4 and f.sub.1, ATF integrated circuit 11 generates tracking error signal ATF-ER having a value determined by a ratio between an amount of a signal having a frequency component f.sub.4 and an amount of a signal having a frequency component f.sub.1. The tracking error signal ATF-ER generated in ATF integrated circuit 11 is shown in FIG. 2E in which reference level V.sub.ref is 2.5 V.
Microprocessor 13 controls driving of a capstan motor 15 based on tracking error signal ATF-ER applied from ATF integrated circuit 11. In more detail, microprocessor 13 samples a particular value of tracking error signal ATF-ER at a sampling position shown in FIG. 2E when tracking error signal ATF-ER is applied thereto. Here, the sampling position shown in FIG. 2E is an intermediate position of each pulse of the head switching signal HEAD-SWP. When the sampled value has a value different from the reference level V.sub.ref, microprocessor 13 controls driving of capstan motor 15 so that the detected value has the same value as that of reference level V.sub.ref. By the operation of the FIG. 1A apparatus, the magnetic tape is precisely transported during double speed reproduction.
However, when the number of times which the magnetic tape has been used increases, the recording state of the magnetic tape deteriorates. When performance of a video cassette recorder used for a playback operation is lowered, an image signal reproduced from the magnetic tape and displayed on a screen picture may quality which is less than satisfactory.